Mercury is a known environmental hazard and leads to health problems for both humans and non-human animal species. Approximately 50 tons per year are released into the atmosphere in the United States, and a significant fraction of the release comes from emissions from coal burning facilities such as electric utilities. To safeguard the health of the public and to protect the environment, the utility industry is continuing to develop, test, and implement systems to reduce the level of mercury emissions from its plants. In the combustion of carbonaceous materials, it is desirable to have a process wherein mercury and other undesirable compounds are captured and retained after the combustion phase so that they are not released into the atmosphere.
One of the most promising solutions for mercury removal from flue gas is Activated Carbon Injection (ACI). Activated carbon is a highly porous, non-toxic, readily available material that has a high affinity for mercury vapor. This technology is already established for use with municipal incinerators. Although the ACI technology is effective for mercury removal, the short contact time between the activated carbon and the flue gas stream results in an inefficient use of the full adsorption capacity of the activated carbon. Mercury is adsorbed while the carbon is conveyed in the flue gas stream along with fly ash from the boiler. The carbon and fly ash are then removed by a particulate capture device such as an Electrostatic Precipitator (ESP) or baghouse.
In flue gas streams containing high concentrations of sulfur oxides, mercury removal by the injection of activated carbon is often compromised by the preferential adsorption and retention of the sulfur compounds such as sulfur trioxide, which are strongly adsorbed by carbon sorbents. The concentration of sulfur dioxide relative to mercury in a typical flue gas stream can be one million to one or higher, and the concentration of sulfur trioxide in such flue gas streams are typically one thousand to one. For example, high sulfur flue gas streams may contain from about 500 parts-per million per volume (ppmv) to about 2500 ppmv or more sulfur dioxide and from about 2 ppmv to about 20 ppmv or more sulfur trioxide. Water vapor in the flue gas further compounds the problem by combining with sulfur trioxide to form sulfuric acid in the pores of the carbon, effectively excluding the adsorption and removal of mercury. For utilities that burn bituminous coals or mixtures of bituminous coals with lower rank sub-bituminous coals, the presence of high levels of sulfur oxides, especially sulfur trioxide, can be a significant concern.
In addition to sulfur oxides that form during coal combustion, sulfur trioxide may also be produced inadvertently in selective catalytic reduction (SCR) processes downstream of the boiler for controlling NO emissions, or it may be added to the flue gas to enhance the performance of ESP devices used to capture the fly ash. Regardless of its origins, sulfur trioxide may have unintended consequences beyond its interference with mercury removal that affect the performance and profitability of the power plant. These consequences include corrosion of system components and unwanted increases in plume visibility and duration upon discharge from the stack.
To prevent the interference of sulfur oxides with mercury removal by the injected mercury sorbent, a number of prior art solutions have been proposed wherein gross reductions in total sulfur oxide levels are achieved in the gas phase. Nearly all of these solutions rely upon bulk injections of alkaline or other reactive agents into the flue gas to react chemically with the sulfur oxides, forming salt particulates in the gas phase which do not usually interfere with mercury adsorption by the sorbent. In some cases, the agent is injected as a dry solid (Dry Sorbent Injection (DSI)), while in other methods an aqueous solution of the agent is injected, which rapidly devolatizes at the temperature of injection to form a very fine, dry powder with even higher reactivity toward sulfur oxides in the duct. For example, trona, a naturally-occurring mixture of sodium carbonate and sodium bicarbonate, is a commercially-available material found to be effective in controlling sulfur oxides when injected into flue gas streams as a dry reactant.
Other alkaline agents, such as calcium oxide (lime), calcium hydroxide (slaked lime), calcium carbonate (limestone), magnesium carbonate (dolomite), magnesium hydroxide, magnesium oxide, and sodium carbonate are also utilized to control sulfur oxide emissions. Solution injection of such agents is represented by Codan's SBS Injection™ technology, which uses an aqueous solution of the chemical reductants sodium bisulfite and sulfite, and is more selective and effective for sulfur trioxide removal. Alternatively, solutions of sodium carbonate, bicarbonate, or hydroxide or thiosulfate can also be used. However, all of these materials and methods suffer disadvantages in that relatively large amounts of the agent must be used for effective control and, more importantly, separate injection systems must be installed independent of mercury sorbent injection, adding cost and complexity to their application. In the case of alkali-based agents, a further disadvantage is found in the negative impact of such materials on the properties of the fly ash collected for subsequent sale to the cement and concrete industry. Although this disadvantage is avoided by using alkaline earth-based agents, these agents generally impart an unwanted increase in resistivity to the ESP, while the alkali-based agents usually have minimal impact on ESP operation.
Where alkaline or other SOx reactive agents have been incorporated into the pore structure of the sorbents themselves, the intent has been uniformly the removal of the sulfur compounds and not the removal of mercury in the presence of such compounds. Numerous other examples of activated carbons and other sorbents that incorporate SOx-reactive materials within the body of the sorbent have been reported, but none appear to advance the art of mercury removal since they are neither directed to that purpose nor are they likely to offer a preferred solution since major portions of the pore structure available for mercury adsorption are configured preferentially for sulfur oxide removal.
There is a need to provide dry sorbent compositions for mercury removal in flue gas streams containing high concentrations of sulfur oxides, especially sulfur trioxide, that do not depend on the independent injection of alkaline or other reactive agents elsewhere in the system for effective mercury removal, and are inherently effective in a single injection mode. Where such alkaline or reactive agents are used as part of the dry sorbent compositions, there is a further need to limit the impact of these agents on balance-of-plant operations by using only what may be necessary to enhance mercury removal locally at the point of sorbent injection, as well as to avoid incorporation within the body of the porous sorbent to afford increased opportunity for mercury removal. Where independent injection of said alkaline or reactive agents may yet be necessary, there is also a need to reduce the amount of such agents that might otherwise be used, consistent with effective mercury removal and marginal impacts on balance of plant issues.